Fatigue crack growth and arrest under high-cycle thermal loading using XFEM in presence of weld residual stresses

摘要

Cracks have been observed under thermal fatigue in some components made of stainless steel in PWR nuclear plants. This paper focuses on the effects of weld residual stresses (WRS) on fatigue crack growth, using Paris' law and extended Finite Element Method (XFEM). A thin pipe with a wall thickness of about 10 mm is considered. The loading is a combination of mean stress (Sm), WRS and a variable amplitude loading (VA) due to thermal fluctuations. Simulations are performed with software Code_Aster. Concerning crack growth without WRS, different parametric studies have been carried out. Crack growth rate of fully circumferential cracks subjected to a representative fraction of the VA signal is compared with that for semi-elliptical cracks (c/a = 10 or c/a=2). Moreover two cycle counting methods have been compared, using different applied thermal loading sequences. Relative to crack growth in presence of WRS: Welding simulation of 14 passes in 2D axisymmetric on a pipe with a chamfer was conducted to determine the WRS which is used as an initial stress field for crack growth studies. Four cracks with an initial length of 0.6 mm positioned at different heights in the pipe at the area of local effect of WRS have been considered. The results show that: in absence of crack growth threshold, important differences in crack growth rates are detected between cracks beginning propagation under small compressive WRS and the ones beginning under tension WRS. It seems possible to explain why some cracks already stopped at 2 mm depth while the others propagate deeper by using the stress intensity factor associated with (Sm+WRS) for different axial crack positions.